Electrical submersible pump (ESP) motors can be very long and small in diameter. To obtain sufficient horsepower, multiple rotor sections are mounted on the same shaft inside the stator with support bearings between each rotor section and on each end of the rotor stack. These bearings consist of a journal bearings with a sleeve mounted on and rotating with the shaft and a corresponding carrier bearing placed over the sleeve and in frictional engagement with the inside diameter of the stator. A thrust washer is usually placed on opposite ends of the rotor carrier bearing between the carrier bearing and the adjacent rotor sections.
The rotor stack is supported at its lower end by a retainer on the shaft. A thrust bearing supports the shaft at the top end of the motor. When the motor heats up, the rotor shaft elongates in a downward direction due to thermal expansion, while the rotor sections simultaneously thermally expand upward along the shaft. The amount of relative motion depends on the properties of materials involved, the rotors being dominated by copper and the shaft by steel.
The thermal growth can result in relative movement of the carrier bearings with respect to the stator. Under certain temperature profiles, the rotor carrier bearings are forced to move axially with respect to the motor stator housing. This movement may occur when the motor experiences large temperature changes when energized, such as in cool subsea or cool well applications, or when the motor is placed in very hot conditions, such as in steam heated viscous oil well applications. This axial movement of the carrier bearings relative to the stator results when axial forces from the spinning rotor sections press against the thrust washers, which in turn press against one end of the carrier bearings, forcing them to move axially relative to the stator to accommodate the thermal expansion discussed above.
Great care must be taken to ensure the carrier bearings can move with sufficient ease as to not impose excessive forces on the thrust washers and at the same time not be so loose a fit in the stator as to cause the carrier bearings to spin within the stator or cock and jamb when being moved. These two conflicting requirements require very precise fitting of the carrier bearings to the inside of the stator and even a change of less than 0.001″ can be the difference between a successful design and motor failure.